A wide variety of mental and physical processes are controlled or influenced by neural activity in particular regions of the brain. For example, the neural functions in some areas of the brain (i.e., the sensory or motor cortices) are organized according to physical or cognitive functions. Several areas of the brain appear to have distinct functions in most individuals. In the majority of people, for example, the areas of the occipital lobes relate to vision, the regions of the left inferior frontal lobes relate to language, and particular regions of the cerebral cortex appear to be consistently involved with conscious awareness, memory, and intellect.
Many problems or abnormalities can be caused by damage, disease and/or disorders in the brain. Disorders include neuropsychiatric and/or neuropsychological disorders, such as major depressive disorder (MDD). A person's neuropsychiatric state may be controlled by particular cortical structures, subcortical structures, and/or signal pathways between such structures.
Neurological problems or abnormalities are often related to electrical and/or chemical activity in the brain. Neural activity is governed by electrical impulses or “action potentials” generated in neurons and propagated along synaptically connected neurons. When a neuron is in a quiescent state, it is polarized negatively and exhibits a resting membrane potential typically between −70 and −60 mV. Through chemical connections known as synapses, any given neuron receives excitatory and inhibitory input signals or stimuli from other neurons. A neuron integrates the excitatory and inhibitory input signals it receives, and generates or fires an action potential when the integration exceeds a threshold potential. A neural firing threshold, for example, may be approximately −55 mV.
When electrical activity levels are irregular, action potentials may not be generated in a normal manner. For example, action potentials may be generated too frequently, or not frequently enough. Such irregularities can result in neurologic dysfunction. It follows, then, that neural activity in the brain can be influenced by electrical energy supplied from an external source, such as a waveform generator. Various neural functions can be promoted or disrupted by applying an electrical current to the cortex or other region of the brain. As a result, researchers have attempted to treat physical damage, disease and disorders in the brain using electrical or magnetic stimulation signals to control or affect brain functions.
Transcranial electrical stimulation (TES) is one such approach that involves placing an electrode on the exterior of the scalp and delivering an electrical current to the brain through the scalp and skull. Another treatment approach, transcranial magnetic stimulation (TMS), involves producing a magnetic field adjacent to the exterior of the scalp over an area of the cortex. Yet another treatment approach involves direct electrical stimulation of neural tissue using implanted deep brain stimulation electrodes (DBS). However, the foregoing techniques may not consistently produce the desired effect with the desired low impact on the patient. For example, TES may require high currents to be effective, which may cause unwanted patient sensations and/or pain. TMS may not be precise enough to target only specific areas of the brain. Deep brain stimulation is a relatively invasive procedure, and it can be relatively difficult to implant DBS electrodes in tissue located well below the cortex. Accordingly, there exists a need for providing more effective, less invasive treatments for neuropsychiatric and neuropsychological disorders.